1. Field of the Invention
This invention relates generally to a polyaxial locking spinal fixation device. More particularly, the present invention relates to a hook and coupling element assembly for attaching to the posterior lamina portion of the spine and for polyaxially receiving and holding securely a rod of an implantation apparatus using same.
2. Discussion of the Prior Art
The bones and connective tissue of an adult human spinal column consist of an upper portion having more than 20 discrete bones, and a lower portion which consists of the sacral bone and the coccygeal bodies. The bones of the upper portion are generally similar in shape, as will be more fully described hereinbelow with respect to FIGS. 1 and 2. Despite their similar shape, however, they do vary substantially in size in accordance with their individual position along the column and are, therefore, anatomically categorized as being members of one of three classifications: cervical, thoracic, or lumbar. The cervical portion, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the 5 lumbar vertebrae.
These bones of the upper portion vary in size, but are each similarly coupled to the next by a tri-joint complex. The tri-joint complex consists of an anterior disc and the two posterior facet joints; the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. Referring now to FIGS. 1 and 2, top and side views of a typical vertebral body of the upper portion of the spinal column is shown. The spinal cord is housed in the central canal 10, protected from the posterior side by a shell of bone called the lamina 12. The lamina 12 has three large protrusions, two of these extend laterally from the side ends thereof and are referred to as the transverse process 14. The third extends back and down from the center of the lamina and is called the spinous process 16.
The anterior portion of the spine comprises a set of generally cylindrically shaped bones which are stacked one on top of the other. These portions of the vertebrae are referred to as the vertebral bodies 20, and are each separated from the other by the intervertebral discs 22. Pedicles 24 are bone bridges which couple the anterior vertebral body 20 to the corresponding lamina 12 and posterior elements 14,16.
The lower portion of the spinal column, which extends into the hip region is primarily comprised of the sacral bone. This bone is unlike the other bones of the spinal column, in both shape and size. In fact, at birth humans have five distinct sacral bones which begin to fuse together during childhood, and by adulthood have fully combined. For the purpose of describing this invention, however, the sacral bone shall be referred to as distinct from the spinal column; the spinal column, therefore, comprising for the purposes of this description, only the cervical, thoracic, and lumbar vertebrae.
In its entirety, the spinal column is highly complex in that it houses and protects critical elements of the nervous system which have innumerable peripheral nerves and arterial and veinous bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants are attached to the back of the spinal column, generally by coupling to the pedicles via screws, or by means of hooks which attach under the lamina and entering into the central canal. In either case, the implants generally comprise elongate support rod elements which are coupled to the screws or hooks to immobilize several sequential vertebrae, for example to hold them stable so that the adjacent bones may be fused with bone graft. The prior co-pending application, U.S. Ser. No. 08/502,285, of which this application is a continuation-in-part, discloses novel devices which provide significantly superior performance for such implants which comprise screws for coupling to the pedicles. The present invention relates to corresponding implant apparatus which utilize hooks.
Such hook and rod assemblies generally comprise a plurality of hooks having rounded blade portions which are inserted posteriorly under the lamina between the transverse process and the spinous process, and upper body portions to which the support rod is coupled. The rod extends along the axis of the spine, coupling to each of a plurality of hooks via receiving portions of their bodies. The aligning influence of the rod forces the spine to which it is affixed, to conform to a more proper shape.
It has been identified, however, that a considerable difficulty may be associated with inserting hooks under sequential lamina along a misaligned curvature and simultaneously exactly positioning their rod receiving portions such that they are aligned so that the rod can be passed therethrough without distorting, tilting, rotating, or exerting undesired translational forces on the hooks. Correction of this difficulty requires the time consuming and difficult task of reshaping the rods or repositioning the hooks, each of which is understood to require considerably longer operating time, which is known to increase the incidence of complications associated with surgery. Often such alignments with such fixed body hooks cannot be satisfactorily achieved, and the entire instrumentationing effort has to be altered to utilizing screws.
It is, therefore, the principal object of the present invention to provide a lamina hook having a rod coupling body which provides a polyaxial freedom of implantation angulation with respect to rod reception.
In addition, it is an object of the present invention to provide such an assembly which comprises a reduced number of elements, and which correspondingly provides for expeditious implantation.
Accordingly it is also an object of the present invention to provide an assembly which is reliable, durable, and provides long term fixation support.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.